Stretch forming of metal sheets provided with reenforcing ribs



Aug. 23, 1960 T. A. CAMERON EIAL 2,949,667

STRETCH FORMING OF METAL SHEETS PROVIDED wrm REENFORCING RIBS Filed Feb.2?, 1958 i 0 INVENTORS. 8 FIG-3 THOMAS A. CAMERON HOWARD F KEYSER'ATTORNEY Patented Aug. as, 1960 STRETCH FORMING OF METAL SHEETS PRO-VIDED WITH REENFORCING '5 Filed Feb. 27, 1958, Ser. No. 717,920

4 Claims. (Cl. 29-428) This invention relates to the provision ofarcuately contoured reenforced metal sheets, especially to a method ofstretch forming initially flat metal sheets having a series ofreenforcing ribs thereon.

Heretofore there have been many articles made wherein it is necessary toprovide a contoured metal sheet that has reenforcing ribs applied tolocal areas thereof. Normally these contoured metal sheets have beenshaped to a desired contour, such as a single or double curvature form.Then reenforcing ribs are separately contoured to a shape complementaryto that of the metal sheet and are secured thereto by means of specialassembly jigs, fixtures or the like. Of course, the initial sheets andthe separately formed reenforcing ribs can each be shaped by stretching,rolling, hydropress, or equivalent shaping operations.

When one endeavors to rivet or weld contoured reenforcing ribs to acontoured or arcuate shaped metal sheet, frequently such securingactions produce localized distortions in the composite structureproduced which is undesirable.

It is the general object of the present invention to avoid and overcomethe foregoing and other difliculties with the above-mentioned processesand to find and provide a new method of stretch-forming metal sheetswith sep arate reenforcing members secured thereto when the sheet is inthe original flat condition and with the sheet and reenforcing membersbeing stretched as an integral unit.

Another object of the invention is to provide an improved, low costmethod of stretch-forming metal sheets to single or double curvaturesand wherein the reenforcing ribs have a symmetrical section, such as a Tor hat section.

A further object of the invention is the provision of a method of thecharacter described wherein the reenforcing ribs have end portions whichare flattened out and secured to the sheet. Preferably, but notnecessarily, such ribs approximately equal in flattened width the widthof the base sheet to which they are secured.

The foregoing and other objects and advantages of the invention will bemade more apparent as the specification proceeds.

Reference now should be had to the accompanying drawings whichillustrate the principles of the present invention and wherein:

Fig. 1 is a plan, partially broken away and shown in section, of areenforced sheet of the invention prior to the contouring thereof;

Fig. 2 is a fragmentary side elevation indicating the first step ofstretch-forming the reenforced sheet of Fig. 1, the sheet beingpartially shown in section; and

Fig. 3 is a fragmentary perspective of the sheet of Fig. 1 after beingstretch-formed, a portion of the sheet being shown in section.

When referring to corresponding parts of the apparatus shown in theaccompanying drawings and referred to in the specification,corresponding numerals are used to facilitate comparison therebetween.

The present invention, generally speaking, relates to a method ofstretch-forming a metal sheet to a desired curved contour and includesthe steps of securing a plurality of reenforcing ribs to one surface ofthe sheet, which ribs have center sections bridging over and spaced fromparts of the sheet, but with the ribs being secured to the sheet at bothlateral margins of the ribs, flattening the ends of the ribs against thesheet, which ribs are of such flattened length as to abut against theedges of adjacent ribs and with the ends of the ribs being secured tothe sheet, and lastly stretching and forming the unit of a sheet andribs so that the metal in the ribs and sheet is stretched beyond itselastic limit, but not beyond its ultimate tensile strength so that aunitary curved article of a desired contour is formed.

With reference to the details of the method of the present invention,attention particularly is directed to the drawings wherein a flat metalsheet 1 is shown which has a plurality of reenforcing ribs 2 securedthereto in parallel relation to each other. These ribs 2 are all ofhat-shape in section, or they could be termed inverted U-shape insection. The ribs 2 have a pair of edges 3 provided thereon and suitablemeans secure the ribs 2 to the sheet l in substantially parallel butspaced relationship. Usually these edges 3 are provided with a pluralityof spot welds or rivets at closely adjacent, longitudinally spacedportions thereof so that the ribs 2 for the purposes of the inventionare made into a unit with the sheet 1 for stretching and formingoperations as hereinafter described.

Either before the ribs 2 are secured to the sheet 1 or at least beforethe end portions thereof are secured to the sheet, these ribs areflattened or spread out laterally as indicated at 4 in Fig. l of thedrawings. Such flattened ribs 4 are likewise secured, as by closelyadjacent spot welds, to the sheet 1 and with the flattened ribs 4abutting edges of adjacent ribs at the end portions of the sheet.

As the next operation is forming the desired contoured unit of theinvention, the unit made from the sheet 1 and ribs 2 is secured in apair of holding or clamping jaws 5 to secure or anchor the edge portionsof the sheet 1 in a desired position. A suitable forming die 6, which ispositioned on a ram 7, is connected to suitable means (not shown) formoving the die 6 longitudinally by means of the ram '7 to bring the die6 into forming engagement with the sheet and rib unit. Thus such sheetand rib unit initially is in fiat or planar form but the axial movementof the die 6 will deform such sheet to a double curvature shape, orotherwise as provided on the working surface of the die 6. This die 6may have any conventional lubrication means associated with its workingface before it is brought into engagement with the sheet 1. Also,minimum tensional forces are set up on the sheet and rib unit by theclamping jaws 5 to tension such sheet slightly and insure that it is inits flattened or straight condition before the die 6 is brought intoengagement therewith.

After the die 6 has been brought into full working engagement with thesheet 1 and rib unit, the metal forming the sheet 1 and ribs 2 has beendeformed to the desired contour. However, normally it is necessary tostretch the shaped metal to insure that the metal in the sheet and ribsis stretched beyond its elastic limit, but not beyond its ultimatetensile strength by the forming or shaping operations performed thereon.Hence, suitable pressures are applied to the clamping jaws 5 to movethem apart in an axial direction and this stretches the metal, such asabout 2 /2% of its original length, to insure that the metal in thesheets and ribs is deformed beyond its elastic limit and will maintainthe desired contour or shape given to it.

After the formed article, indicated at 8 in Fig. 3 of the drawings, hasbeen removed from engagement with the clamping jaws 5, it may bedesirable to trim oif the flattened end portions 4 of the ribs 2. Thusthe unit 8 could be cut off or have the end sections removed asindicated in the vertically sectioned portion of the unit 8 of Fig. 3.

By practice of the present invention, relatively large structures can beformed to a desired contour and such articles will retain their givenshapes within reasonable limits, such as A3 inch or less.

While the present invention has been shown with the use of a so-calledhat shaped reenforcing rib therein, it will be realized that anysuitable type of a reenforcing rib can be used wherein some type of abridged or overhanging center portion is provided in the rib and withthe tWo edge portions on each rib being secured to the base sheet byeither cementing, welding, riveting or other similar operations.

Preferably when providing the reenfo-rcing ribs for the base sheet inthe unit to be stretched, it is desirable that the rib have slightlythinner gauge metal used therein inasmuch as such rib portion of themetal unit must be stretched or pulled farther than the base sheet andwill thus yield a little more readily than it the base sheet andreenforcing ribs were all made of the same thickness. The use ofunsymmetrical reenforcing angles of L or Z shape, for example, whereinonly one edge of the reenforcing member is secured to the base sheet ofthe unit is possible, particularly if the curved contours are notsevere. However, it is generally preferable to use symmetricalreenforcing shapes of T or hat shape, for example.

It also should be noted that the present invention can be used with anyconventional ductile metal such as aluminum, stainless steel, or otherconventional materials. It should be realized that the ultimate use ofthe contoured metal unit provided by the invention will determine theparticular shape and size of the reenforcing ribs used thereon and thegauge of material therein with relation to the gauge of the metal in thebase sheet. Thus for stiffening action, a light, but relatively highcrowned or center portion should be provided on the reenforoing rib toprovide the desired result. For other installations, it may be desiredto use a relatively wide, low height reenforcing rib.

It also will be realized that the present invention particularly lendsitself to the production of quite large metal units such as large metalreflectors for use in radar appa ratus and in the telephone industry.These reflectors may be 7, 8 ft. or more long and may be 2, 3 ft. ormore wide. These units normally must be held to a tolerance of plus orminus /s inch, for example.

While a certain representative embodiment and details have been shownfor the purpose of illustrating the invention, it will be apparent tothose skilled in this art that various changes and modifications may bemade therein without departing from the spirit or scope of theinvention.

What is claimed is:

1. A method of stretch forming a flat metal sheet unit to a desiredcurved contour comprising securing a plurality of substantially parallelstraight reenforcing ribs each having pairs of parallel continuousattaching flanges for engagement with the sheet and continuous spacedstanding flanges substantially normal to one surface of the sheet withthe ribs extending in a direction of desired curvature, each rib havinga center section spaced from and bridging over a part of said sheet andsecured to said flanges that are substantially normal to said sheet atthe edges thereof remote from said sheet, the ends of such ribs beingflattened against said sheet and substantially abutting against theedges of adjacent ribs, the ends 1 of such ribs being secured to saidsheet, gripping opposite ends of the assembled members at the flattenedrib ends, tensioning the flat assembly in a direction parallel to thereenforcing ribs to stretch the unit by about 2%.% and forming the unitof said sheet and ribs to a double curvature form to a degree so thatthe metal in the ribs and in the sheet is stretched beyond its elasticlimit and to the dmired curvature but not beyond its ultimate tensilestrength and without collapse of said ribs.

2. A method of stretch forming a flat metal sheet unit to a desiredcurved contour comprising securing a plurality of substantially parallelstraight reenforcing ribs each having pairs of continuous attachingflanges for engagement with the sheet and continuous standing flangesnormal to the sheet to one surface of the sheet with the ribs extendingin a direction of desired curvature, said ribs having center sectionsspaced from and bridging over parts of said sheet and being secured tosaid sheet at both of the lateral margins of such ribs, the ends of suchribs being flattening against said sheet, said ribs being made fromthinner gauge material than said sheet, gripping opposite ends of theassembled members at the flattened rib ends, tensioning the flatassembly in a direction parallel to the reenforc'ing ribs to stretch theassembled unit, and forming the unit of said sheet and ribs to a doublecurvature so that the metal in the ribs and in the sheet is stretched bythe tensioning and forming beyond its elastic limit to the desiredcurvature but not beyond its ultimate tensile strength.

3. A method as in Claim 2 where said sheet and rib unit is stretched byabout 2 /2% by the tensioning step.

4. A method as in claim 1 where the flattened ends of said ribs and theportions of said sheet thereadjacent are trimmed from the unit after theforming thereof.

References Cited in the file of this patent UNITED STATES PATENTS1,462,474 Atkinson July 24, 1923 2,263,425 McKee Nov. 18, 1941 2,386,018Watter Oct. 2, 1945 2,415,240 Fouhy Feb. 4, 1947 2,527,412 Green Oct.24, 1950 2,636,968 Watter Apr. 28, 1953 2,638,958 Norquist et a1 May 19,1953

